Tyrosine hydroxylase has been purified to homogeneity from cultured rat phechromocytoma cells (PC 12). The purification involves precipitation with ammonium sulfate (30-42%), Sepharose CL-6B, DEAE, hydrophobic interaction chromatography, and Fractogel HW55(P) gel filtration. The enzyme has an oligomeric molecular weight of 230,000 and is composed to four similar subunits of 60,000 molecular weight. The enzyme incorporates phosphate when incubated with 32p-ATP, Mg2+, and the catalytic subunit of protein kinase.